A high-voltage discharge lamp is used as a lamp for a light source such as a projector, the high-voltage discharge lamp being configured such that 0.2 mg/mm3 or more of mercury is sealed in an inside of a transparent glass discharge vessel and the pressure of the vessel is 200 atmosphere or higher at the lighting time. The light in a visible wavelength region can be obtained with a high output by making the mercury vapor pressure high.
FIGS. 10A and 10B each show a schematic sectional view of the high-voltage discharge lamp. FIG. 10B is a schematic sectional view obtained by enlarging the vicinity of a leading end of an electrode in FIG. 10A.
As shown in FIG. 10A, a high-voltage discharge lamp 10 has a light emitting part 11 which is formed by a discharge vessel and has a substantially spherical shape. A pair of electrodes 20a and 20b are arranged in the light emitting part 11 so as to be opposite to each other with an extremely small distance of 2 mm or less.
Further, sealing part 12 are formed at both ends of the light emitting part 11. Metal foils 13 for an electrical conduction are buried in an airtight manner in the sealing parts 12, and shaft parts (30a and 30b) of the electrodes 20a and 20b are bonded to one ends of the metal foils 13. Further, external leads 14 are bonded to the other ends of the metal foils 13, and power is supplied from a power supply part (not shown).
In the high-voltage discharge lamp 10 described above, protrusions 21 are formed respectively in tip side surfaces of a pair of electrodes 20a and 20b which are arranged in an opposite manner in the light emitting part 11 of the light emitting tube, during the lighting, and a discharge arc 22 is retained between the protrusions 21. As a result, a stable lighting state is maintained (refer to FIG. 11A).
On the other hand, in the case that the high-voltage discharge lamp 10 is lighted in the same state for a long period, a plurality of minute protrusions 23 may be formed due to a high temperature, or minute irregularities are generated in the tip surface of the electrodes (refer to FIG. 11B). The minute protrusions 23 and the irregularities are generated by the melting of a material (for example, tungsten) constituting the electrodes 20a and 20b, and the aggregation of a chemical compound generated by being bonded to gas which is sealed within the light emitting part 11, and this existence changes a shape of the surface of the tip of the electrode. It has been known that a starting point of an arc moves in conjunction with this, the discharge position becomes unstable, and an illuminance reduction and a flickering are generated.
In order to solve the problem described above, the following Patent Document 1 discloses an illumination system of a discharge lamp which supplies a current waveform of pulse waves P1 having a predetermined frequency (a fundamental frequency) to a high-voltage discharge lamp and which intermittently or periodically inserts a current waveform of pulse waves P2 having a lower frequency than the fundamental frequency to the pulse waves having the fundamental frequency (refer to FIG. 12). More specifically, the fundamental frequency is set to a frequency which is selected from a range between 60 and 1000 Hz, and the frequency having a lower frequency is set to a frequency which is selected from a range between 5 and 200 Hz. Further, the control is performed so as to increase or decrease the time for supplying the fundamental frequency little by little every predetermined time. In other words, this system changes the generation frequency of the pulse waves having the low frequency in response to the time.
A period that one electrode is fixed to an anode and the other electrode is fixed to a cathode, that is, a period that a high voltage is applied between both the electrodes becomes longer by setting the frequency of the pulse waves to the low frequency. As a result, a heating degree applied to the electrode is enhanced, and it is possible to transmit the heat not only to the tip of the electrode, but also to a position which is away from the tip. Therefore, the heat is transmitted to the position which is away from the tip of the electrode during the application of the pulse waves having the low frequency, and it is possible to melt and/or evaporate the minute protrusions or the irregularities which are generated at the positions. As a result, it is possible to eliminate the protrusions and the irregularities in the other positions than the tip of the electrode which may adversely affect, rather than the protrusions to be an arc starting point.